Traditional locomotives are known to use several on-board electrical systems to drive an output or load. Similarly, other electronic devices such as portable hand-held or wireless devices include load-driving circuitry. Typically, such electrical systems and devices include mechanisms to limit the amount of current that may be supplied to the load or output circuit. One such mechanism is a current limiter, which limits the current that may be supplied to the load or output circuit.
A conventional current limiting circuit 200 is illustrated in FIG. 2 that limits the current supplied to load 201. Circuit 200 uses the base-to-emitter voltage, or VEB, of PNP transistor Q1 to limit the current when load 201 attempts to draw current that exceeds a predetermined limit value. As the load 201 draws increasing current, the VEB of transistor Q1 increases from 0 mV to 650 mV based on the voltage drop across resistor RSENSE. A VEB of 650 mV begins to “turn on” the PNP transistor Q1, which increases the voltage drop across resistor RGATE. This increase in the voltage across resistor RGATE begins to “turn off” PMOS (or p-type MOSFET) Q2 because the gate voltage of PMOS Q2 begins to increase. This turning off of Q2 continues until the current limit is reached. The above configuration actively limits the load current in this manner. The arithmetic expression of the current limit value (ILIM) at room temperature may be given by:ILIM=VBE÷RSENSE≈0.650V÷RSENSE  (1)
Equation (1) above shows that one may select a current limit value by means of selecting an appropriate RSENSE resistor. In FIG. 2, circuit 200 limits the current deliverable to load 201 at approximately 217 mA (217 mA≈650 mV÷3Ω).
Circuit 200, however, suffers from variation over temperature, since the VEB of Q1 depends upon temperature for a given emitter-base current. An emitter-base current that produces a VEB of 650 mV at 25° C. produces approximately 800 mV at −40° C. and 420 mV at 150° C. That is, the threshold VEB value at which Q1 “turns on” increases with a decrease in temperature. It will be apparent from equation (1) that such a change in the VEB threshold value from 650 mV at room temperature will result in a large variation in the current limit for load 201. Specifically, the load current will limit at too high for low temperatures, and too low at high temperatures.
U.S. Pat. No. 5,587,649 discloses a scheme that recognizes the variation in the VEB threshold value for transistor Q1 in a current limiting circuit. The '649 patent suggests replacing sense resistor RSENSE with a combination of resistors including a thermistor having a negative temperature coefficient.
While the '649 patent may disclose a current limiting circuit that may take into account the variation in the VEB threshold value for transistor Q1, the disclosed current limiting circuit does not attempt to maintain a low variation in the current limit over a wide temperature range. Instead, the disclosed current limiting circuit assumes that the maximum current demand of load 201 changes over temperature and the current limit tracks this change in current demand over temperature. Accordingly, the disclosed current limiting circuit may not be able to provide a low variation in the current limit over a wide temperature range.
Further, the disclosed current limiting circuit may not be useful for systems in which a high load current is required. This is because commercially available thermistors have a large resistance value as a result of which the combination of resistors including the thermistor will have a large effective resistance. As a result, a low load current (of the order of milli-amps) will cause a large voltage drop across the resistor combination sufficient to “turn on” transistor Q1, thereby limiting the load current to the milli-amps range.
The presently disclosed current limiting circuit and system including the same is directed to overcoming one or more of the problems set forth above and/or other problems in the art.